So, what happened with this? (LoL on Steam)

It was a patch update for those who had purchased it on Steam a long time ago when it wasn't f2p

"For a brief period of time after League of Legends launched in 2009, players could purchase a digital collectors edition of the game," Cole said. "This was distributed through a variety of channels (Steam included) via our European partners. We ended those distribution arrangements in 2010, but a small number of players still have this version of the game in their Steam inventories, so that's why it occasionally pops up."

Although Steam does support free-to-play games, and LoL is one of the most popular F2P titles in existence, a Steam release for that game would've been odd. Valve, the owners of Steam, have a similar game in development called Dota 2. If Valve wants to challenge Riot's control of the MOBA genre, it seems unlikely that they'd provide that competing game with direct access to the Steam community.

Riot distributes LoL directly through the game's official website. Cole added in the Reddit thread that the company has no plans to change that business model.